Low profile waveguide channel diplexer

ABSTRACT

A waveguide channel diplexer in which plural resonant rectangular cavities are coupled to a segmented circular waveguide through thin slots on a plane longitudinal surface of the guide. The slots are located to preserve circular symmetry and consequently the purity of the circular electric mode; the cavities which produce a complementary bandpass-bindstop filter pair are designed to be electroformed from permanent mandrels.

[451 June 6,1972

United States Patent Tuchen S m E M w mm A M. RD E m U m C m m A w m m on LD M Miller........................................333/9 3,321,720 5/1967 Shimada...........................333/2lRX Murray Hill, NJ.

Apr. 16, 1971 Primary Examiner--Paul L. Gensler Attorney-R. .1. Guenther and E. W. Adams, Jr.

221 Filed:

[57] ABSTRACT A waveguide channel diplexer in which plural resonant [21] Appl. No.:

[52] US. Cl...............................333/6, 333/21 R, 333/73 W,

rectangular cavities are coupled to a segmented circular 333/83 R noi 5/12 ....333/6, 9, 21 R, 21 A, 73 w,

waveguide through thin slots on a plane longitudinal surface of [51] Int. Cl.

the guide. The slots are located to preserve circular symmetry [58] Field olSearch.........

and consequently the purity of the circular electric mode; the

333/95 R, 98 R cavities which produce a complementary bandpass-bindstop filter pair are designed to be electroformed from permanent mandrels.

7 Claims, 2 Drawing Figures PATENTEDJUH 6 I972 SHEET 10F 2 ATTORNEY LOW PROFILE WAVEGUIDE CHANNEL DIPLEXER BACKGROUND OF THE INVENTION This invention relates to electromagnetic waveguide transmission and more particularly to waveguide mode transducing devices, especially a complementary bandpass-bandstop filter which may be used for channel dropping or channel combining in a millimeter wave transmission system.

The TE circular electric mode of wave propagation in circular waveguides has received considerable attention due to its recognized low-loss propagation characteristic which exhibits decreasing loss with increasing frequency. The problem of multiplexing in a communication system using this mode of wave propagation was first considered by E. A. J. Marcatili in his article entitled Mode-Conversion Filters," published in the January 1961 issue of the Bell System Technical Journal, pages 149-184. The filters described by Marcatili in this article, and in his US. Pat. No. 2,963,663, use high loss-narrow bandwidth coaxial cavities.

The high loss of a coaxial cavity, which is formed from rectangular waveguide wrapped around a cylinder such as a circular waveguide wall, causes the overall filter loss to considerable if many such cavities are used. Some of the filters utilize circular electric mode conversion resonators to achieve channel separation. These exhibit an extremely low loss, but provide only a single bandpass section, and for multipole arrangements, additional bandpass sections of the coaxial cavity type are required. This, of course, introduces substantial loss and severe bandwidth limitations.

Coupling to the coaxial cavity is provided through a distributed arrangement of apertures in the circumferential wall. Due to the circular geometry, it is difficult and costly to fabricate the cavities and the appropriate aperture arrangement. In addition, both the mode conversion resonator and the coaxial resonator exhibit relatively narrow frequency bands which are free from spurious resonances. In certain transmission systems, it is essential to have channel diplexers capable of operating over as broad a bandwidth as possible and many system applications require a band broader than these cavities provide. Improvements which would increase bandwidth and simplify the structural arrangement would make the device more attractive. 1

The desirable TE mode, which exhibits a longitudinal component that is exclusively magnetic (H and is therefore alternatively designated the 1-1 circular orI-I mode, may be propagated in a segmented circular waveguide. As used herein, a segmented circular waveguide means a waveguide having a cross-section which is a segment of a circle, such as a semicircle, and the resulting chordal surface is the plane surface containing the chord of the cross-sectional segment, or the diameter of the segment is a semicircle. The longitudinal magnetic component H is maximum at the center line of the chordal surface, and it is thus possible to couple directly to this mode at this central location. There have been previous attempts to couple to a plane surface of a segmented waveguide, such as is disclosed in US. Pat. No. 3,112,460 to S. E. Miller. However, these arrangements require bifurcating a circular guide into two parallel semicircular guides and coupling equal amounts of incident energy into each. Both of the parallel semicircular guides are coupled to the output port with critical dimensions arranged so that the half of the energy arriving at the output guide from one of the semicircular guides is outof-phase with the other half of the energy arriving from the other semicircular guide, thereby preventing any of the incident energy from coupling to the output guide. A frequency selective reflecting device in each of the semicircular guides causes energy at the selected frequency to return to the apertures and a 180 phase delay must be provided in one guide to insure the desired coupling to the output port. This complex arrangement is, of course, difiicult to fabricate.

SUMMARY OF THE INVENTION In accordance with the present invention, a channel diplexer is provided which ofiers fabrication simplification and broader bandwidth than the previous filter designs without significantly sacrificing the low-loss characteristics of the circular electric mode propagation. The main through waveguide is a single segmented waveguide, and cavities are mounted directly on the chordal surface. The cavities which are preferably complementary bandpass and band rejection filters, are tuned to resonate at a specified channel frequency. Coupling to the longitudinal magnetic component is made along the center line of the chordal surface by means of a single aperture for each cavity. Since all the coupling apertures are located on a plane surface, rather than in the more complex circumferential distribution required to couple to coaxial cavities, a substantial cost reduction over that type of filter is realized; there is also a substantial reduction over the cost of the parallel semicircular arrangement.

A segmented circular guide exhibits a low through loss characteristic similar to that of a circular guide. Through loss in a circular waveguide is slightly lower than that in a segmented guide of the same diameter, but the loss in the segmented guide is still effectively negligible and is a small price to pay for the large increase in bandwidth offered by the segmented structure. In one typical system application, the input wave in the main waveguide could havea percentage bandwidth greater than 15 percent. These bandwidths are substantially higher than can be provided if the main waveguide were circular in cross-section.

The present invention is an improved version of the channel diplexer claimed in the copending application of C. Ren and H. Wang, Ser. No. 134,805, filed on an even date herewith and assigned to the assignee hereof. The improved diplexer is envisioned as part of a tandem arrangement of numerous similar diplexers in a millimeter wave multiplexer. It may, of course, have other applications.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of a channel diplexer in accordance with the present invention; and

FIG. 2 included for purposes of explanation, is an exploded view of an electroformed embodiment of the diplexer of FIG. 1.

DETAILED DESCRIPTION FIG. 1 illustrates a two-pole channel diplexer in accordance with the invention comprising a two-cavity branching filter 20 and a two-cavity band rejection filter 30 longitudinally disposed along segmented waveguide 10. Waveguide 10 is proportioned to support a continuous range of frequencies in thel-l mode, including channel bands centered at f,, f, f,,. Hereinafter, this range is referred to as the subband, and each channel is designated by its center frequency.

Segmented circular waveguide 10 is conductively bounded by circumferential wall 11 and plane surface 12. In order to preserve the circular symmetry of the electric and magnetic field patterns propagating in waveguide 10 and hence avoid unwanted moding, the waveguide is designed so that the chordal surface 12 contains the diameter of the circular cross-section; i.e., waveguide 10 is a semicircular waveguide.

In a millimeter wave multiplexer, a large number of channel diplexers are arranged in tandem. In such a system it is preferable to have as wide a subband bandwidth as possible and if the diameter of waveguide 10 is properly chosen a percentage bandwidth (free of cut-off for any mode) which is approximately 18 percent may be established. The subband is located between the cut-off frequencies of the higher order circular modes H and E3 It is noted that the circular magnetic mode E020 would prevent such a wide bandwidth; this mode is present in circular guides, but is not supported by the segmented guide. Therefore, the subband propagating in waveguide 10 is wider than that in a circular guide.

The H fmode propagates in the segmented waveguide with an extremely 1owloss characteristic. The magnitude of the loss is slightly greater in a segmented guide than a circular guide of the same diameter, due to the additional loss from the induced surface currents on the flat chordal surface. The increased loss is, however, extremely small and even if it were on the order of twice as large, the channel diplexer loss would still be insignificant relative to the loss provided by the more conventional rectangular waveguide.

The diplexer is shown and described as a channel dropping filter with the subband signal f ,f f}, entering Input Port 1, the dropped channel of frequency f} propagating out at Output Port 2, and the remainder of the subband signal after the dropped channel has been removed propagating out at hrough Port 3, but the diplexer is, of course, reciprocal and may also be used as a channel combiner.

The incoming subband signal containing a combination of caannels propagates in the H fmode. The channel which is to be dropped is coupled into branching filter 20. Branching filter 20, together with the two-cavity band rejection filter 30, forms a two-pole filter of complementary design. Thus, the through channels f f f jg f,, will propagate down waveguide 10 under matched conditions.

Branching filter 20 contains two rectangular cavities 21 and 22, which are end-coupled by inductive coupling through their mutual wall 23. Rejection filter 30 contains two rectangular cavities 31 and 32. Cavities 21, 31 and 32 are longitudinally coupled to waveguide 10 through coupling apertures 16, 17 and 18, respectively, located on chordal surface 12. Each cavity must have its magnetic field component aligned with the thin coupling slot in the common wall between the cavity and surface 12. These coupling apertures are oriented along the center longitudinal axis 13 of the chordal surface so that the presence of these apertures preserves the circular symmetry of the structure with respect to the fields of the incoming waves of H mode. In order to avoid moding problems, the apertures are slots which are thin relative to the diameter of waveguide 10. These slots, which may be rectangular or elliptical, couple only energy from the circular H mode to the cavities. But since H,,,,,frequencies are below cut-off for m a 2, only H will excite the cavities, and the diplexer will not respond to incoming signals in any spurious modes.

The longitudinal magnetic field in waveguide is maximum along center line 13 of surface 12, and since thin slot apertures 16, 17 and 18 are located along this line, strong coupling to the longitudinal component of the cavity resonant mode is provided. The magnetic field at this location is, for example, several times larger than the longitudinal magnetic field of the H mode in the circular waveguide on its circumferential waveguide wall.

Branching filter 20 and band rejection filter are dimensioned so that their cavities resonate at j}, the center frequency of the channel to be dropped. This is accomplished by choosing suitable cavity dimensions as described hereinafter. Thus, the desired channel f, is coupled to Output Port 2 and the rejection cavities turn back energy in this frequency band, preventing its passage to Through Port 3. The bandpass and band rejection cavities are designed as a complementary pair and each has the conventional maximally flat frequency response characteristic within the operating subband, so that constant resistance will be seen looking into Input Port 1.

Any oversized (overrnoded) waveguide may be used for the resonance cavities to achieve a higher intrinsic Q for the resonant mode which may be either the dominant or higher order mode. However, if the cavity dimensions are large enough to allow higher order modes, frequencies in the subband f f}, outside the desired channel band f, may be resonant for such higher modes. Therefore, for broad subband bandwidth, the cavity dimensions should be chosen so that no spurious resonance exists at frequencies within the subband. The dominant l-l mode will provide the widest operating bandwidth, but higher order H Emodes may be attractive for certain applications, particularly where low loss is more important than greatest subband bandwidth.

The cavities are positioned so that the narrow dimension w of the cavity cross-section lies on surface 12 and is perpendicular to the axis of waveguide 10; the broad cross-sectional dimension I is perpendicular to surface 12. The length of each cavity is designated h, and it is oriented parallel to the propagation in waveguide 10. For a resonant frequency of f, and lowest loss, the cavity length must be approximately equal to n times the dimension 1 which, in turn, must be approximately equal to A l V2: where n is the order of the resonant H Emode and A, is the free-space resonance wavelength at f In FIG. 1 the most downguide" rejection cavity 32 is shown having a length '1 approximately twice the length of rejection cavity 31. Cavity 32 supports the H mode, while cavity 31 supports the H 5 mode. Likewise, bandpass filter 20 may be dimensioned for higher order modes and cavity 21 has a length 11,, substantially equal to 11 The second bandpass cavity 22 also supports the H mode and has a length it, which is approximately equal to h,,. It is positioned upguide" from and inductively coupled through their mutual end wall 23 to cavity 21. The dominant mode may be used throughout the structure but the higher order modes result in lower loss.

Coupling slots 16, 17 and 18 are located on center line 13 of surface 12 and separated by a center-to-center distance L which is an odd multiple of one-quarter M; ii, is the guide wavelength of the H mode in waveguide 10. The coupling slot is centrally located on the length h of a dominant mode cavity such as 31 and the increased length of the higher order cavities, such as 21 and 32, does not alter the relative location of the slot and the nearest end wall of the cavity. Cavity 22 is coupled to waveguide 10 only through cavity 21.

The energy at frequency in the resonant mode may be coupled from cavity 22 to Output Port 2 in any physical manner so long as it is compatible with the electrical design and causes no moding problems. The simplest form is an inductive coupling through end wall 24. It is anticipated that output waveguide 26 will continue the narrow dimension w of the cavities for a significant distance and guide 26 may thus be oversized in width and require a transducer for the conversion to standard waveguide. Waveguide 26 which is beyond the diplexer region may, of course, be turned to facilitate system design.

Since all of the cavities lie on surface 12 and extend above surface 12 by the same broad dimension 1, the diplexer of FIG. 1 is referred to as having a low (or horizontal) profile in distinction to the diplexers having varying heights above surface 12, such as are disclosed in the aforementioned copending application of Ren and Wang. The low profile offers a number of significant mechanical advantages. Primarily, the diplexer can be fabricated by a simple electroforming process which requires that any protrusions and indentations of the mandrel be shallow to facilitate deposition in the concave mandrel recesses. FIG. 2 is an exploded view of the electroformed components of the low profile diplexer.

The two components, an upper portion 40 and a lower portion 50 form the cavities, and due to the selection of a suitable mating plane in the low profile geometry and the use of inductive obstacles 47, 48, 57 and 58, instead of other coupling means the two mandrels used for electroforming can be readily extracted and, therefore, may be of the permanent type. High profile designs require destruction of the mandrels (referred to as lost" mandrels), and hence different production models of those diplexers are subject to variations. The permanence of the mandrels for the low profile design make possible precise reproduction of all diplexers for the same channel frequency. As can be seen, the two electroformed portions 40 and 50 combine to produce, in addition to the resonance cavities 21, 22, 31 and 32, all of the coupling apertures 16, 17 and 18 and the output waveguide 26 as shown in FIG. 1. In addition, access openings for tuning, such as 41, 42 and 43 may be provided in upper portion 40 and alignment holes, such as 55 in the lower portion 50 and corresponding holes (not shown) in the underside of upper portion 40, can also be generated. Tuning openings 41, 42 and 43 may be of any desired shape suitable for tuning elements, such as a capacitive screw or preferably a magnetic field displacement vane. It is noted that the electroforming process may produce a channel for output waveguide 26 which is considerably longer than is shown and in addition this output channel may be turned upward into portion 40 to form a vertical output port.

The rectangular geometry, particularly that having constant narrow dimensions w and t, facilitates extremely precise, and economical mandrel fabrication by means of profile grinding. The inductive end walls 23 and 24 are grown in," that is, an additional piece of material is inserted into precisely located and precisely machined slots in the mandrel after it is cleaned so that the resulting electroformed portions 40 and 50 include protrusions 47 and 57, and 48 and 58 which combine to form conductive walls 23 and 24, respectively.

The mass precision production of all of these elements nakes fabrication substantially simpler and less costly than other manufacturing processes required for diplexers of other geometries.

The low profile geometry yields additional advantages. Since the permanent mandrels do not need to be dissolved, they may be formed of a stable material such as an austenitic stainless steel. Without other constraints the permanent mandrel material may be chosen to resist the corrosive attack of a plating solution, such as sulfuric acid, which will produce a high conductivity electroformed product. The cavity walls of a low profile diplexer can thus have higher conductivity than is attainable using the lost mandrels required to produce high profile designs.

The low profile geometry also allows mating plane 60 to be located midway between the narrow walls of the cavities and hence along a line which is not crossed by electric currents in the desired mode.

If the broad cavity dimension I is to be approximately 100 mils, the upper portion 40 would be approximately 100 mils thick with 50 mil indentations forming the upper halves of the cavities. The thickness of portion 40 contributes physical strength to the diplexers. Similarly, the lower portion 50 would have 50 mil indentations, but the remaining part of portion 50 may be machined down to form a surface 65 which is separated from the narrow cavity walls by a very small distance such as mils. This ground surface 65 then forms the chordal plane of segmented waveguide 10. The circumferential wall 11 can be fonned by electroforming a block 66 around a lost mandrel which is secured to ground plane 65 and appropriately aligned with apertures 16, 17 and 18. The electroforming will cause block 66 to fuse to surface 65.

After fusing surface 65 to block 56 the upper and lower portions 40 and 50 are readily sealed along mating plane 60 by tapped holes and screws not shown. Alignment holes,'such as 55, and their corresponding holes in portion 40, insure precise alignment of the cavities. A commercially available round pin, such as 63, is inserted between the corresponding holes which may be square to facilitate production by the electroforming process. Gasket 61 fits in its channel 62, which may be machined in upper portion 40. This provides a good pneumatic seal which is required if a dry gas (such as nitrogen) is to be maintained under pressure within the diplexer structure. A commercially Any diplexer, in which rectangular cavities are mounted on a chordal surface of a segmented waveguide so that one wall of each cavity lies in a single common plane located a fixed distance from the chordal surface, has a low profile design and offers the aforementioned mechanical advantages and the resultant economies of fabrication.

In all cases it is to be understood that the above-described arrangements are merely illustrative of a small number of the many possible applications of the principles of the invention. Numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. A waveguide transmission device comprising a conductivity bounded waveguide having a curved surface and a single plane surface and capable of propagating energy in a circular electrode mode, a bandpass cavity capable of supporting a resonant mode and mounted on the single plane surface of the waveguide, and means comprising a single aperture in the single plane surface for exclusively coupling energy between the circular electric mode in the waveguide and the resonant mode in the bandpass cavity, said bandpass cavity being a rectangular prism positioned so that the narrow cross-sectional dimension of the bandpass cavity lies on the single plane surface perpendicular to the axis of the waveguide.

2. A waveguide transmission device comprising, a segmented circular waveguide capable of supporting the H circular electric mode and having a circumferential surface and a plane chordal surface parallel to the axis of the waveguide, a primary bandpass resonant cavity of rectangular cross-section having selected narrow and broad dimensions and mounted on the chordal surface, a pair of band rejection resonant cavities of rectangular cross-section having said selected narrow and broad dimensions and mounted on the chordal surface downguide from the primary bandpass cavity, at least one thin aperture for the primary bandpass cavity and for each band rejection cavity located on the chordal surface for coupling between the H circular electric mode of the waveguide and the resonant mode of the cavity, a secondary bandpass cavity having selected narrow and broad dimensions and mounted on the chordal surface, said secondary bandpass cavity being positioned upguide from said primary bandpass cavity longitudinally end-to-end with the primary bandpass cavity and coupled thereto through their mutual end wall, all of said cavities having a dimension chosen so that each cavity supports a resonant H rectangular mode in a common frequency band, and an output port coupled to said secondary bandpass cavity 3. A waveguide transmission device as claimed in claim 2 wherein the narrow cross-sectional dimension of each cavity lies on the chordal surface perpendicular to the axis of the waveguide.

4. A waveguide transmission device as claimed in claim 2 wherein said segmented circular waveguide has a semicircular cross-section.

5. A waveguide transmission device as claimed in claim 2 wherein the length of each cavity is selected so that n 2 for at least one of said cavities.

6. A waveguide transmission device as claimed in claim 2 wherein each of said cavities are mounted so that one wall of each cavity lies in a single common plane located a fixed distance from the chordal surface.

7. A waveguide transmission device as claimed in claim 2 wherein said cavities and the output waveguide have a common narrow dimension w and common narrow dimension t of the coupling apertures. 

1. A waveguide transmission device comprising a conductivity bounded waveguide having a curved surface and a single plane surface and capable of propagating energy in a circular electrode mode, a bandpass cavity capable of supporting a resonant mode and mounted on the single plane surface of the waveguide, and means comprising a single aperture in the single plane surface for exclusively coupling energy between the circular electric mode in the waveguide and the resonant mode in the bandpass cavity, said bandpass cavity being a rectangular prism positioned so that the narrow cross-sectional dimension of the bandpass cavity lies on the single plane surface perpendicular to the axis of the waveguide.
 2. A waveguide transmission device comprising, a segmented circular waveguide capable of supporting the H01 circular electric mode and having a circumferential surface and a plane chordal surface parallel to the axis of the waveguide, a primary bandpass resonant cavity of rectangular cross-section having selected narrow and broad dimensions and mounted on the chordal surface, a pair of band rejection resonant cavities of rectangular cross-section having said selected narrow and broad dimensions and mounted on the chordal surface downguide from the primary bandpass cavity, at least one thin aperture for the primary bandpass cavity and for each band rejection cavity located on the chordal surface for coupling between the H01 circular Electric mode of the waveguide and the resonant mode of the cavity, a secondary bandpass cavity having selected narrow and broad dimensions and mounted on the chordal surface, said secondary bandpass cavity being positioned upguide from said primary bandpass cavity longitudinally end-to-end with the primary bandpass cavity and coupled thereto through their mutual end wall, all of said cavities having a dimension chosen so that each cavity supports a resonant H10n rectangular mode in a common frequency band, and an output port coupled to said secondary bandpass cavity.
 3. A waveguide transmission device as claimed in claim 2 wherein the narrow cross-sectional dimension of each cavity lies on the chordal surface perpendicular to the axis of the waveguide.
 4. A waveguide transmission device as claimed in claim 2 wherein said segmented circular waveguide has a semicircular cross-section.
 5. A waveguide transmission device as claimed in claim 2 wherein the length of each cavity is selected so that n 2 for at least one of said cavities.
 6. A waveguide transmission device as claimed in claim 2 wherein each of said cavities are mounted so that one wall of each cavity lies in a single common plane located a fixed distance from the chordal surface.
 7. A waveguide transmission device as claimed in claim 2 wherein said cavities and the output waveguide have a common narrow dimension w and common narrow dimension t of the coupling apertures. 